A method of this nature is for example known from DE 36 22 926 A1. In the method described therein a flat metal sheet is passed through a rolling mill with a central working roller with a very large diameter and a plurality of second working rollers with a significantly smaller diameter, which are arranged on the external circumference of the first working roller. The second working rollers are arranged one behind the other and have one or more protrusions on their external circumference, wherein the width of these protrusions of the consecutively arranged second working rollers increases. In this way the flat metal sheet is reshaped such that it assumes a special cross-sectional shape with one or a plurality of longitudinal grooves or channels on one side.
It is known that during the manufacture of a metal sheet of this nature undulations can be formed in the metal sheet. This formation of undulations should be reduced through the combination of the first central working roller with a large diameter with the plurality of second working rollers with a significantly smaller diameter, which are arranged on the first central working roller. In addition, retaining plates can be provided, which are arranged between the second working rollers and press the metal sheet in this region onto the external circumference of the central working roller, so that the metal sheet in these regions does not bulge outwards.